1. Field of the Invention
The present invention relates to a method of manufacturing a carcass structure for tyres for vehicle wheels.
The invention also relates to a carcass structure for tyres for vehicle wheels obtainable by the above method, said carcass structure comprising; at least one carcass ply formed of strip-like sections each of which extends in a substantially U-shaped conformation following the cross-section outline of the tyre and comprises one or more thread-like elements longitudinally arranged parallelly of each other and preferably at least partly coated with a layer of raw elastomer material; and a pair of annular reinforcing structures each engaged close to a respective inner circumferential edge of the carcass ply.
2. Description of the Related Art
Manufacture of tyres for vehicle wheels involves formation of a carcass structure essentially made up of one or more carcass plies substantially having a toroidal conformation and presenting their axially opposite side edges engaged to respective annular, circumferentially inextensible, reinforcing elements usually referred to as “bead cores”.
Applied to the carcass structure, at a circumferentially outer position, is a belt structure comprising one or more belt strips having the shape of a closed ring, which are essentially made up of textile or metal cords suitably oriented relative to each other and to the cords belonging to the adjacent carcass plies.
Then, at a circumferentially outer position of the belt structure a tread band is applied which usually consists of a strip of elastomer material of suitable thickness.
It is to point out that, to the aims of the present description, by the term “elastomer material” it is intended a rubber blend in its entirety, i.e. the assembly formed of a base polymer suitably amalgamated with mineral fillers and/or process additives of various types.
Ultimately, a pair of sidewalls is applied to the opposite sides of a tyre being manufactured, each of said sidewalls covering a side portion of the tyre included between a so-called shoulder region, arranged close to the corresponding side edge of the tread band, and a so-called bead arranged at the corresponding bead core.
The traditional production methods essentially provide that the above listed tyre components should be first made separately of each other, to be then assembled during a manufacturing step of the tyre.
For example, for making the carcass ply or plies to be associated with the bead cores in order to form the carcass structure, it is first required that, by an extrusion and/or calendering process, a rubberized fabric comprising longitudinally-disposed continuous textile or metal cords should be produced. This rubberized fabric is submitted to a transverse cutting operation to produce sections of predetermined lengths that are subsequently joined together so as to give origin to a continuous ribbon-like semifinished product, having transversely-arranged parallel cords.
This manufactured article must then be cut into sections the length of which is correlated with the circumferential extension of the carcass to be produced.
Manufacturing methods have been also proposed in which, instead of resorting to production of semifinished articles, the carcass structure is made directly during the tyre manufacturing steps.
For example, U.S. Pat. No. 5,453,140 herein referred to as an example of the most pertinent state of the art, discloses a method and an apparatus forming a carcass ply starting from a single cord that has been previously wound up on a reel.
In accordance with the method and apparatus described in the above patent, at each working cycle of the apparatus the cord taken up from the reel by power-driven pulling rollers and maintained taut by a pneumatic tensioning system is cut to size so as to obtain a section of a predetermined length.
The cord section is taken up by a grip element mounted on a belt looped around power-driven pulleys to be transversely laid down on the outer surface of a toroidal support.
The section ends are then engaged by folding members of the belt type operation on opposite sides of the toroidal support to radially apply the cord section to the toroidal support itself by slider elements acting like fingers along the section side portions.
Repetition of the above described working cycle leads to deposition of these cord sections in a circumferential side-by-side relationship until the whole circumferential extension of the toroidal support is covered.
Necessarily, the toroidal support is previously coated with one or more raw rubber layers having a dual function, that of adhering to the cords laid down thereon so as to conveniently hold them in a fixed positioning, and that of forming an inner air-proof layer in the finished tyre.
Tyres obtained by this manufacturing method have a carcass structure in which the carcass ply or plies are formed of individual cords each having two side portions axially spaced apart from each other and radially oriented relative to the rotation axis of the tyre, and a crown portion extending at a radially outer position between the side portions.
Within the scope of the carcass structure manufacture, it is also known that close to each of the tyre beads, the opposite ends of the individual cords forming a carcass ply are arranged in an alternating sequence, at axially opposite positions relative to an annular anchoring element forming said bead core, having the shape of a crown made up of radially-overlapping thread or wire coils, as can be viewed from Patent EP 0 664 231 and U.S. Pat. No. 5,702,548.
In the above mentioned art all cords forming the carcass ply or plies however, are substantially arranged in the neutral axis of resistance to bending of the respective bead. Under this circumstance, the structural resistance of the beads must necessarily rely on the stiffness of the filling inserts of very hard elastomer material incorporated into the bead structure, the behaviour of which feels the effects of temperature changes due both to environmental factors and to stresses produced during normal operation.
In Patent FR 384 231 it is proposed to make a carcass structure by deposition on a toroidal support, of a series of rectangular small bands of rubberized fabric disposed circumferentially in side-by-side relationship and arranged in radial planes relative to the geometric axis of the support drum itself. Deposition of the small bands is carried out in such a manner that the end flaps of two non-consecutive small bands are partly covered with the end flaps of the small band interposed therebetween. Spaces existing between the end flaps of the covered small bands are filled with trapezoidal inserts applied to the end flaps of the small band placed in overlapping relationship thereon. Deposition of the small bands is executed in different overlapped layers, the number of which is correlated with the thickness to be given to the carcass structure. The presence of said trapezoidal inserts gives rise to thickening of the carcass structure at the bead regions, where it has a thickness which is twice that found at the crown.
In U.S. Pat. No. 4,248,287 it is disclosed a method according to which formation of the carcass structure involves that a plurality of layers each formed of radial strips consisting of rubberized threads and circumferentially disposed in side by side relationship should be laid down on a toroidal drum. When deposition has been completed, at the bead region two bead cores are applied and the end flaps of the carcass layers formed by the radial strips are then turned back around them.
The Applicant has found that important advantages can be achieved, both in terms of simplification of the production processes and in terms of improvement of the behavioural features of the tyre, if the carcass ply or plies are manufactured by conveniently laying down strip-like sections, each comprising a plurality of cords parallel to each other incorporated into an elastomer layer, onto a rigid toroidal support.
In this connection the Applicant has already developed several manufacturing methods being the object of respective European patent applications.
For instance, in European patent applications No. 97830731.2 and No. 97830733.8 a manufacturing method and a tyre are respectively described in which the carcass structure is obtained by making a first and a second carcass plies each accomplished by strip-like sections laid down sequentially in a circumferential side-by-side relationship.
Tyres obtained as described in these patent applications have the end portions of the strip-like sections belonging to the first and second carcass plies disposed on respectively opposite sides relative to the annular reinforcing structures of the beads.
This expedient, in combination with the respectively crossed orientation of the strip-like sections belonging to one and the other carcass plies, offers important advantages in terms of structural resistance of the tyre close to the beads and the sidewalls.
In the European patent application No. 98830472.1, in the name of the same Applicant as well, accomplishment of a carcass ply is proposed which is carried out by depositing a first and a second series of strip-like sections in alternated sequence, in which the sections belonging to the first and second series terminate at respectively opposite sides relative to the reinforcing structures of the beads.
Advantages can be thus achieved in terms of structural resistance at the tyre beads and sidewalls even in the presence of a single carcass ply.